Clay–Carbon Nanotubes Hybrid Materials for Nanocomposite Membranes: Advantages of Branched Structure for Proton Transport under Low Humidity Conditions in PEMFCs

A new class of hybrid materials based on carbon nanotubes (CNT) rooted on smectite clays (SWy) was synthesized by catalytic chemical vapor deposition (CCVD) method, and studied to be introduced in a perfluorosulfonic acid (Nafion) membrane. Side-wall chemical oxidation and organo-functionalization of the CNT was performed using organic ester molecules containing hydrophilic groups (−RSO₃H). SWy–CNT nanoadditives were incorporated in the polymer by solution-precipitation method producing highly homogeneous nanocomposite membranes with outstanding mechanical properties. Materials were characterized by a combination of techniques (TGA, Raman, FT-IR, SEM, TEM, and DMA), while a deep investigation on the water transport properties was performed by NMR methods (PFG and relaxation times). Membranes containing SWy–oxCNT–RSO₃H nanoadditives are able to guarantee a very high proton diffusion in “quasi-anhydrous” conditions. Proton mobility is ensured by a correct network created from the long nanotubes (well distributed through the clay nanoplatelets) appropriately functionalized with acid groups. Remarkable are the electrochemical results: the best membrane reaches conductivities of 7 × 10^–2 S cm^–1 at 120 °C and 30% RH, 1 order of magnitude higher than pristine polymer, and a rather high value in the current panorama of the PEMFCs